1. Field of the Invention
The present invention relates to a method for balancing a shaft-and-wheel assembly of a turbocharger.
2. Description of Related Art
Turbochargers are provided on an engine to deliver air to the engine intake at a greater density than would be possible in a normal aspirated configuration. This allows more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight.
Generally, turbochargers use the exhaust flow from the engine exhaust manifold, which enters the turbine housing at a turbine inlet, to thereby drive a turbine wheel, which is located in the turbine housing. The turbine wheel is affixed to one end of a shaft, wherein the shaft drives a compressor wheel mounted on the other end of the shaft. As such, the turbine wheel provides rotational power to drive the compressor wheel and thereby drive the compressor of the turbocharger. This compressed air is then provided to the engine intake as described above.
The compressor stage of the turbocharger comprises the compressor wheel and its associated compressor housing. Filtered air is drawn axially into a compressor air inlet which defines a passage extending axially to the compressor wheel. Rotation of the compressor wheel forces pressurized air flow radially outwardly from the compressor wheel into the compressor volute for subsequent pressurization and flow to the engine.
Turbocharger efficiency has been increased by the adoption of rolling element bearings (REB) to support the rotating assembly that includes the turbine wheel, the compressor wheel, and the connecting shaft. For example, there is an improvement in transient response of the turbocharger due to the reduction in power losses, especially at low turbocharger RPM, of the REB system over some typical turbocharger bearing systems such as hydrodynamic sleeve-type bearing systems. REB systems can also support much greater thrust loads than can typical turbocharger bearing systems, making the thrust component more robust. However, REBs have a finite fatigue life which is sensitive to the loads being transmitted through them. The bearing loads are dependent on the level of imbalance of the wheels, so the fatigue life of the REB is sensitive to the imbalance of the turbine and compressor wheels. There exists, therefore, a need to ensure that the imbalance levels of the turbine and compressor wheels are as low as possible during operation of the turbocharger.